The present invention relates to polishing and planarizing of surfaces, such as semiconductor wafer surfaces, and more particularly, to fixed abrasive pad and platen arrangements for chemical-mechanical polishing/planarizing of semiconductor wafers.
In the process of fabricating modem semiconductor integrated circuits (ICs), it is necessary to form various material layers and structures over previously formed layers and structures. However, the prior formations often leave the top surface topography of an in-process wafer highly irregular, with bumps, areas of unequal elevation, troughs, trenches, and/or other surface irregularities. These irregularities cause problems when forming the next layer. For example, when printing a photographic pattern having small geometries over previously formed layers, a very shallow depth of focus is required. Accordingly, it becomes essential to have a flat and planar surface, otherwise, some parts of the pattern will be in focus and other parts will not. In fact, surface variations on the order of less than 1000 angstroms over a 25xc3x9725 mm die would be preferable. In addition, if the irregularities are not leveled at each major processing step, the surface topography of the wafer can become even more irregular, causing further problems as the layers stack up during further processing. Depending on the die type and the size of the geometries involved, it is desirable to effect some type of planarization, or leveling, of the IC structures. In fact, most high-density IC fabrication techniques make use of some method to form a planarized wafer surface at critical points in the manufacturing process.
One method for achieving semiconductor wafer planarization or topography removal is the chemical-mechanical planarization/polishing (CMP) process. In general, the CMP process involves holding the wafer against and moving the wafer relative to a polishing platen under a controlled pressure or friction force. A typical CMP apparatus includes a polishing head for holding the semiconductor wafer against the polishing platen. The polishing platen is covered with a pad. The pad is usually either an open cell foamed polyurethane (e.g. Rodel IC1000) or a sheet of polyurethane with a grooved surface (e.g. Rodel EX2000). The pad surface is wetted with a polishing slurry containing chemicals and, optionally, abrasive particles. This pad typically has a backing layer called a sub-pad that interfaces with the surface of the platen.
Another type of polishing pad is the xe2x80x9cfixed-abrasive padxe2x80x9d made from abrasive particles fixedly dispersed in a suspension medium. Unlike conventional CMP slurries, the planarizing liquid employed with a fixed abrasive pad may be free of abrasive particles, i.e., a polishing chemical solution. Chemical-mechanical polishing/planarization with a chemical solution on a fixed abrasive pad offers several desirable aspects: low chemical cost, low dishing, and a good top surface finishing.
One concern with metal CMP is the heat generated during the polishing process, caused by chemical reaction of the metal with the polishing chemical and the friction of the wafer against the fixed abrasive pad surface. Heat build-up may degrade the quality of the wafer being polished. Fixed abrasive pads with thick stacks exhibit poor thermal conductivity and, therefore, wafers are not readily cooled during CMP on such pads.
Another concern is the dishing observed generally in metal CMP. Dishing can be high if the pad modulus is low. A conventional fixed abrasive pad may include polishing elements of 40 xcexcm in height that are mounted in a thin Mylar(trademark) sheet. The other layers below the Mylar(trademark) sheet form a sub-pad stack, and may have five layers, for example. A top adhesive layer joins the Mylar(trademark) sheet to the sub-stack. A polycarbonate layer, which is supposed to provide the necessary pad modulus, is underneath the top adhesive layer, and a foam layer, which is supposed to provide long range flexibility, is joined to the polycarbonate layer by a middle adhesive layer. A bottom adhesive layer on the foam layer attaches the polishing pad formed by the Mylar(trademark) sheet and sub-pad stack to the top of the platen. However, due to the existence of the layer of very soft adhesive material between the very thin Mylar(trademark) sheet and the polycarbonate layer, the modulus of the integrated pad of six layers cannot be high enough for strict dishing requirements. Elimination of this adhesive layer would greatly increase the pad modulus and help the pad dishing performance.
A further concern is the consumable cost of fixed abrasive polishing pads. Although only the top Mylar(trademark) layer is subjected to wear during polishing, the entire pad is normally replaced when the Mylar(trademark) pad is excessively worn. This increases the cost of ownership.
Another concern is the system vibration that may occur when polishing with conventional fixed abrasive pads. Such vibrations can occur due to the typical construction of fixed abrasive pads, and their attachment on top of a platen. The foam pad that forms part of the polishing pad is not readily changeable to damp the system vibrations of an individual polisher.
The present invention overcomes problems with existing fixed abrasive polishing pads by providing a platen for a fixed abrasive pad arrangement, comprising a platen housing, a sub-structure integrally mounted on the platen housing, and a mounting arrangement configured to removably mount a fixed abrasive layer to the integral sub-structure.
One of the aspects of certain embodiments of the fixed abrasive pad arrangement of the present invention is that the fixed abrasive layer may be readily replaced since it is removably mounted to the sub-structure, or sub-pad. As the platen has an integral sub-pad, the sub-pad does not have to be replaced whenever the top polishing layer requires replacing. This reduces consumable expenses, as only the top polishing layer is replaced, rather than an entire polishing pad with sub-layers together.
As the sub-structure is integrally mounted on the platen housing, a cooling arrangement may be provided that improves the cooling of the overall sub-structure and fixed abrasive layer, in comparison to conventional fixed abrasive polishing pads that include a sub-structure stack and a fixed abrasive layer. Since the sub-structure is integral to the platen housing in certain embodiments of the invention, a cooling arrangement in accordance with embodiments of the invention may be provided that better cools the sub-structure and the fixed abrasive layer.
An aspect of the present invention provides a chemical-mechanical polishing machine comprising a platen housing, a sub-pad integral with the platen housing, and a mounting arrangement configured to removably mount a fixed abrasive layer to the sub-structure. In certain embodiments of the invention, the mounting arrangement includes a vacuum mounting arrangement that removably and quickly mounts the fixed abrasive layer to the sub-structure, without the use of adhesive. Eliminating the adhesive between the fixed abrasive layer and the sub-structure reduces the undesirable phenomenon of dishing.
The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.